How to interpret Lactate Threshold non-detection?

First: To detect LT you are required to use a chest strap HR sensor, but I guess you have this covered as I assume you won't be able to start the test unless the required sensor is connected.

Second: You say that your top HR measured this year is 189. Is this just the highest value you have observed, or is it the result of a proper MaxHR test? If the guided test is not able to detect your LT you should check the MaxHR setting, as this will affect how the watch will set up your test in terms of HR zones and pace.

Third: I'm not sure how the test work, but since you cancelled the test that might be the reason why it didn't record a result.

I've never done the guided test myself, but usually the watch detects and updates my LT HR & Pace during normal runs with mixed intensity.

Use a HRM strap, and finish the test!

Watches vibrate when I am outside the target HR zone during the test. My assumption was that if I can't keep myself in the target zone I don't meet test requirements.
And so I quit. But yes, next time, I can try to finish the 180+ lap even if I don't meet 180+ requirement.

Please remember, that your heart rate needs some time to rise after you start sprinting. This will cause initial alerts of being below target zone, but it should gradually get there.

MaxHR is the absolute maximum heart rate you are able to reach. There are (lab)tests to find this value, but it's painful and some even argue that it's not recommended to do.
So, the 189 is what you have observed during normal exercise and if that is obtained without going really really all out or feeling the need to throw up or anything - your real MaxHR is probably a few beats above this - I would suggest to set yours at maybe 192 or even 195 and retry the LT test. However - increasing MaxHR will skew your HR zones upwards accordingly, so the test will actually get slightly harder to perform...

Nevertheless - completing the test is probably the first thing to try.

Regarding trail runs; I'm not 100% sure about LT, but at least other metrics like VO2max are calculated only when using the 'Run' app. I assume it's the same for LT as well.

This FirstBeat summary explains pretty well how the LT is determined from the HRV deflection point:

https://www.firstbeat.com/en/consumer-feature/lactate-threshold/

So, to measure it you need:

(a) measure your HRV during exercise (hence the need for an HR strap)
(b) need to go beyond the deflection point

For me, the resulting value has been surprisingly close to blood lactate tests (usually off by no more than 5 bpm or 15 s/km in pace terms - you need to decide whether that is sufficiently exact for your purposes), but you need to understand that this is an estimation and it has been done with HR for a long term and rightfully criticized for a long term (see e.g. http://fellrnr.com/wiki/Heart_Rate_Deflection). However, the deflection is much more pronounced in HRV than in HR, so it might be a good enough approach for many. Especially if you consider that you can do the test however often you want to.

I have still never been able to successfully perform a lactate threshold test. I've tried with a F3HR, FR935, and now with the F5+ and I always have to bail before I can complete the test. I usually get a new threshold notification when doing intervals or a max effort negative split 5k and it seems accurate.

I don’t think there’s any point in doing the guided test as long as you get LT estimates from normal running, results should be the same.

But maybe that was your point Browner40 ?

I will repeat what I have said many times before:
I believe that it is important to smoothly increase your effort up through the range where an LT can be detected. If I do that, I get a new LT every time.

The problem is that the guided test doesn't really communicate this. It is very natural to think that one's effort should increase in steps every time a new interval is started. It is also natural to increase the effort too much from one interval to the next, so it is suddenly necessary to dial down the effort to come back into the specified heart range range.

I believe that this will often cause data of a really poor quality, not very suited for detecting an LT.

So my advice is: Go for a normal run on a road with no elevation change. After a good warmup, increase your pace slowly and steadily over a period of 10-20 minutes until you are well above your assumed LT heart rate. During all of this, don't try to control your heart rate. Only concentrate on smoothly increasing your effort.

Then I am pretty certain that your watch will detect an LTHR.

By the way, you mentioned trail runs with climbs. The Firstbeat algorithms don't use elevation change to calculate their metrics. They only use elevation change to discard a portion of the run session if the elevation change is too large. So if you want something useful from a Firstbeat algorithm, run on a road with no elevation change.

+1 !

If you follow Allan’s instructions and still don’t get a LT value, then you should contact Garmin support.